Li2O—Al2O3—SiO2 based crystallized glass and method for producing same

ABSTRACT

In a Li 2 O—Al 2 O 3 —SiO 2  based crystallized glass using SnO 2  as a substitute fining agent for As 2 O 3  or Sb 2 O 3 , a crystallized glass having less yellow coloration is provided at low costs. The glass is a Li 2 O—Al 2 O 3 —SiO 2  based crystallized glass comprising from 0.01 to 0.9% of SnO 2  in terms of % by mass and having a content of each of As 2 O 3  and Sb 2 O 3  of 1,000 ppm or less as a glass composition, which has a V 2 O 5  content of from 0.08 to 15 ppm in the glass composition.

TECHNICAL FIELD

The present invention relates to a Li₂O—Al₂O₃—SiO₂ based crystallized glass and a method for producing the same. More specifically, it relates to a Li₂O—Al₂O₃—SiO₂ based crystallized glass suitable for heat-resistant applications such as front windows of oil stoves, wood stoves, and the like and top plates for cookers, and a method for producing the same.

BACKGROUND ART

A Li₂O—Al₂O₃—SiO₂ based crystallized glass in which a Li₂O—Al₂O₃—SiO₂ based crystal such as a β-quartz solid solution (Li₂O.Al₂O₃.nSiO₂ [4>n≧2]) or a β-spodumene solid solution (Li₂O.Al₂O₃.nSiO₂ [n≧4]) is precipitated has a low coefficient of thermal expansion and high mechanical strength and hence has excellent thermal characteristics. In addition, it is possible to regulate the kind of a precipitated crystal by appropriately adjusting heat treatment conditions in a crystallization step, so that a transparent crystallized glass (a β-quartz solid solution precipitates as a main crystal) and an opaque crystallized glass (a β-spodumene solid solution precipitates as a main crystal) can be easily produced. Making use of such characteristics, the Li₂O—Al₂O₃—SiO₂ based crystallized glass has been widely used as materials such as front windows of oil stoves, wood stoves, and the like, substrates for high-technology products such as color filter substrates and image sensor substrates, setters for firing electronic components, trays for microwave ovens, top plates of electromagnetic cookers, and window glasses for fire-retarding doors.

Moreover, there has been known a Li₂O—Al₂O₃—SiO₂ based crystallized glass in which a coloration component such as V₂O₅ is added to a Li₂O—Al₂O₃—SiO₂ based crystallized glass to color it dark-brown. The colored crystallized glass hardly transmits a visible light but has a high infrared transmittance, so that the glass has been used as top plates of cookers utilizing infrared rays.

Incidentally, in the case of producing a Li₂O—Al₂O₃—SiO₂ based crystallized glass, it is necessary to perform melting at a high temperature exceeding 1,400° C. Therefore, as a fining agent to be added to a glass batch, As₂O₃ or Sb₂O₃ capable of generating a lot of a fining gas at the time of melting at the high temperature has been used. However, since As₂O₃ and Sb₂O₃ exhibit strong toxicity, attention should be paid so as not to contaminate the environment at the production process of the glass, at the time of treating a waste glass, or at a similar time.

Accordingly, SnO₂ has been proposed as a substitute fining agent for As₂O₃ or Sb₂O₃ (e.g., see Patent Reference 1).

CITED REFERENCES Patent References

Patent Reference 1: JP-A-11-228181

Patent Reference 2: U.S. Pat. No. 4,093,468

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

However, in the case of using SnO₂ as a fining agent, yellowish coloration occurs in the Li₂O—Al₂O₃—SiO₂ based crystallized glass. This coloration is especially problematic in the case of obtaining a transparent crystallized glass.

As a method for suppressing coloration of a transparent crystallized glass, there is a method of adding a colorant having a complimentary color relation to erase color. Particularly, for the Li₂O—Al₂O₃—SiO₂ based crystallized glass, it is hitherto known that Nd₂O₃ is effective for erasing color (e.g., see Patent Reference 2). Therefore, even in the case where the yellowish color becomes strong by the addition of SnO₂, it is possible to erase color by adding Nd₂O₃. However, since the color erasing with Nd₂O₃ is, so to speak, a technology of making the yellow coloration achromatic by overlaying blue coloration with Nd₂O₃, there are problems that transmittance in a visual region decreases as a result, the appearance looks blackish, and a transparent feeling is easily impaired. In addition, since Nd₂O₃ is a rare metal and a batch cost becomes very high, it is impossible to provide an inexpensive Li₂O—Al₂O₃—SiO₂ based crystallized glass.

An object of the present invention is to provide a Li₂O—Al₂O₃—SiO₂ based crystallized glass having less yellow coloration in the crystallized glass using SnO₂ as a substitute fining agent for As₂O₃ or Sb₂O₃.

Means for Solving the Problems

The present inventors made extensive and intensive investigations. As a result, it has been found that (1) when V₂O₅ and SnO₂ coexist, coloration of V₂O₅ is enhanced by SnO₂ and a glass is colored, (2) the coloration caused by V₂O₅ occurs in a minute amount of V₂O₅ to which consideration is usually not necessary, and (3) a considerable amount of V₂O₅ is contained in usual glass raw materials as an impurity, and thus they have proposed the present invention. Incidentally, even when As₂O₃ or Sb₂O₃ is used as a fining agent, the coloration of V₂O₅ does not become strong. Moreover, in a dark-brown crystallized glass, V₂O₅ is used as a coloring agent in the first place, so that the problem itself is not present. Namely, this problem is first revealed by the use of SnO₂ as a fining agent for a Li₂O—Al₂O₃—SiO₂ based crystallized glass intentionally containing no V₂O₅, especially a Li₂O—Al₂O₃—SiO₂ based transparent crystallized glass.

The Li₂O—Al₂O₃—SiO₂ based crystallized glass of the present invention devised in consideration of the above circumstances is a Li₂O—Al₂O₃—SiO₂ based crystallized glass comprising from 0.01 to 0.9% of SnO₂ in terms of % by mass and having a content of each of As₂O₃ and Sb₂O₃ of 1,000 ppm or less as a glass composition, which has a V₂O₅ content of from 0.08 to 15 ppm in the glass composition. In the present invention, the “Li₂O—Al₂O₃—SiO₂ based crystallized glass” means a crystallized glass in which a crystal comprising Li₂O, Al₂O₃ and SiO₂ as essential components is precipitated as a main crystal. The crystal comprising Li₂O, Al₂O₃ and SiO₂ as essential components means a Li₂O—Al₂O₃—SiO₂ based crystal such as a β-quartz solid solution (Li₂O.Al₂O₃.nSiO₂ [4>n≧2]) or β-spodumene solid solution (Li₂O.Al₂O₃.nSiO₂ [n≧4]), for example. Moreover, the “V₂O₅ content in the glass composition” is an amount of total V (vanadium) components contained in the glass composition in terms of V₂O₅. The V components may be intentionally added or may be unavoidably mixed in.

In the present invention, it is preferable to comprise from 55 to 75% of SiO₂, from 10 to 35% of Al₂O₃, from 1 to 10% of Li₂O, from 0.2 to 5% of MgO, from 0 to 5% of ZnO, from 0 to 10% of BaO, from 0 to 4% of TiO₂, from 0 to 5% of ZrO₂, from 0 to 4% of P₂O₅, and from 0.01 to 0.9% of SnO₂, in terms of % by mass, as a glass composition. In the present invention, there is not excluded the case where a component other than the above components is contained.

In the present invention, it is preferable to comprise from 17 to 27% of Al₂O₃ and from 0.2 to 4% of MgO, in terms of % by mass, as a glass composition.

As a cause of mixing of the V₂O₅ impurities, there are mentioned mixing from raw materials and mixing at the glass production process. With regard to the raw materials, it has been found that much one is mixed in from raw materials of Al₂O₃ and MgO. Accordingly, when the above constitution is adopted, the contents of Al₂O₃ and MgO can be limited, so that the amount of mixing of V₂O₅ can be effectively reduced.

In the present invention, it is preferable that Li₂O 0.741MgO+0.367ZnO is from 3.7 to 5.0%, in terms of % by mass, as a glass composition. Incidentally, the “Li₂O+0.741MgO+0.367ZnO” means a value obtained by summing the content of Li₂O, the content of 0.741×MgO, and the content of 0.367×ZnO.

In the Li₂O—Al₂O₃—SiO₂ based crystallized glass, SnO₂ has a function of strengthening coloration caused by not only V₂O₅ but also Fe₂O₃, TiO₂, and the like. For suppressing the coloration, it is suitable to increase the Al₂O₃ amount contained in a remaining glass phase after crystallization. Li₂O, MgO and ZnO tends to precipitate in a crystal phase along with Al₂O₃. Thus, by decreasing the amounts of these components, it becomes possible to reduce the Al₂O₃ amount to be distributed into the crystal phase to distribute more amount of Al₂O₃ into a glass phase. Accordingly, when the above constitution is adopted, the amounts of Li₂O, MgO and ZnO can be restricted to the necessary minimum and the Al₂O₃ amount in the remaining glass phase can be increased.

In the present invention, the Fe₂O₃ content in the glass composition is preferably 250 ppm or less. The “Fe₂O₃ content in the glass composition” is an amount of total Fe (iron) components contained in the glass composition in terms of Fe₂O₃. The Fe components may be intentionally added or may be unavoidably mixed in.

As already mentioned, Fe₂O₃ becomes a cause of coloration similarly to V₂O₅. Accordingly, when the above constitution is adopted, it becomes easy to suppress the coloration caused by Fe₂O₃.

In the present invention, it is preferable that transmittance of a light having a wavelength of 525 nm is 89% or more at a thickness of 1.1 mm. Here, the term “at a thickness of 1.1 mm” means a value obtained by converting a measured value of transmittance in terms of a thickness of 1.1 mm.

In the present invention, it is preferable that the b* value of L*a*b* colorimetric system of CIE standard is from 0 to 4.5 at a thickness of 3 mm. Here, the term “at a thickness of 3 mm” means a measured value of the b*value at a thickness of a measurement sample of 3 mm.

In the present invention, it is preferable that a β-quartz solid solution is precipitated as a main crystal.

When the β-quartz solid solution precipitates as a main crystal, the crystallized glass is liable to be transparent to a visible light. In the case of a transparent crystallized glass, since the coloration by V₂O₅ remarkably influences the appearance, the effect exerted by applying the present invention can be more precisely enjoyed in the case of adopting the above constitution.

Moreover, the method for producing a Li₂O—Al₂O₃—SiO₂ based crystallized glass of the present invention is a method for producing a Li₂O—Al₂O₃—SiO₂ based crystallized glass which comprises preparing a raw material batch so as to form a glass comprising from 0.01 to 0.9% of SnO₂ in terms of % by mass and having a content of each of As₂O₃ and Sb₂O₃ of 1,000 ppm or less, melting and forming the batch to produce a Li₂O—Al₂O₃—SiO₂ based crystallizable glass, and subsequently subjecting the glass to a heat treatment to achieve crystallization, wherein selection of raw materials and management of steps are performed so that a V₂O₅ content in the glass becomes from 0.08 to 15 ppm. Here, the “management of steps” means that the mixing of V₂O₅ from the production process is appropriately managed, for example, contamination of a compounding equipment and a conveying equipment of the raw material batch is prevented.

In the present invention, it is preferable to prepare the batch so as to form a glass comprising from 55 to 75% of SiO₂, from 10 to 35% of Al₂O₃, from 1 to 10% of Li₂O, from 0.2 to 5% of MgO, from 0 to 5% of ZnO, from 0 to 10% of BaO, from 0 to 4% of TiO₂, from 0 to 5% of ZrO₂, from 0 to 4% of P₂O₅, and from 0.01 to 0.9% of SnO₂, in terms of % by mass, as a glass composition. Incidentally, in the present invention, there is not excluded the case where a component other than the above components is contained.

In the present invention, it is preferable that a raw material having a V₂O₅ content of 50 ppm or less is used as a raw material of Al₂O₃. Moreover, it is preferable that a raw material having a V₂O₅ content of 400 ppm or less is used as a raw material of MgO.

As already mentioned, V₂O₅ is largely mixed in from raw materials of Al₂O₃ and MgO. Accordingly, when the above constitution is adopted, the V₂O₅ amount to be mixed in from Al₂O₃ and MgO can be effectively reduced.

In the present invention, it is preferable that melting is performed at a temperature of less than 1,750° C.

As the temperature is elevated, the coloring action of V₂O₅ by SnO₂ is strengthened. Accordingly, when the above constitution is adopted, the coloration caused by V₂O₅ can be effectively suppressed.

In the present invention, it is preferable that a β-quartz solid solution is precipitated as a main crystal by a heat treatment.

When the β-quartz solid solution is precipitated as a main crystal, since the crystallized glass is liable to be transparent to a visible light, the effect exerted by applying the present invention can be more precisely enjoyed in the case of adopting the above constitution.

Effect of the Invention

In the Li₂O—Al₂O₃—SiO₂ based crystallized glass of the present invention, since the V₂O₅ content is strictly restricted, the coloration caused by coloring of V₂O₅ is suppressed although SnO₂ is used as a fining agent as a substitute fining agent for As₂O₃ or Sb₂O₃.

Moreover, according to the method of the present invention, even when an expensive decoloring agent such as Nd₂O₃ is not used, a Li₂O—Al₂O₃—SiO₂ based crystallized glass having no coloration can be produced.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

The following will describe the Li₂O—Al₂O₃—SiO₂ based crystallized glass of the present invention. Incidentally, the term “%” in the following explanation means “% by mass” unless otherwise indicated.

In the Li₂O—Al₂O₃—SiO₂ based crystallized glass of the present invention, the V₂O₅ content is strictly restricted and is reduced to a level where the coloring of V₂O₅ does not cause an influence. The reason why the coloring of V₂O₅ becomes not problematic in the case of using As₂O₃ or Sb₂O₃ and becomes problematic in the case of using SnO₂ is not sufficiently elucidated but the present inventors have surmised as follows.

V₂O₅ and SnO₂ are not incorporated into a crystal and remain in a glass matrix phase. In other words, these components are concentrated in the glass matrix phase. Furthermore, when an oxidation-reduction reaction with V₂O₅ from standard electrode potential is compared, in SnO₂, the oxidation reaction from Sn²⁺ to Sn⁴⁺ is energetically advantageous and V₂O₅ is liable to be reduced, thereby increasing V⁴⁺. In combination of these conditions, even when the content of V₂O₅ is minute, it is considered that the coloration of the glass becomes a non-negligible level. On the other hand, in As₂O₃ or Sb₂O₃, when the oxidation-reduction reaction with V₂O₅ from standard electrode potential is compared as described above, the reduction reaction of V₂O₅ becomes energetically disadvantageous as compared with the case of SnO₂ and thus V₂O₅ is difficultly reduced, so that V⁴⁺ is not increased. Accordingly, in the conventional crystallized glass using As₂O₃ or Sb₂O₃ as a fining agent, it is considered that the coloration by V₂O₅ does not become problematic.

The following will describe a mixing source of V₂O₅. As the mixing of V₂O₅, mixing from the raw materials and mixing at the glass production process are considered.

First, the mixing of V₂O₅ from the glass raw materials will be described. When impurities contained in glass raw materials are analyzed, it has been confirmed that there are raw materials containing much V₂O₅ as an impurity among those widely used as raw materials of Al₂O₃ and MgO. Moreover, there exist those containing a large amount of V₂O₅ also among the other raw materials, for example, those to be used as raw materials of SiO₂, raw materials of ZrO₂, and the like. Therefore, it is desirable to select raw materials each containing less amount of the V₂O₅ impurity, especially raw materials of Al₂O₃ and MgO each having a small content of V₂O₅. Moreover, it is desirable to design a composition having small contents of Al₂O₃ and MgO so that the use amounts of the raw materials of Al₂O₃ and MgO can be reduced as far as possible.

The following will describe the mixing of V₂O₅ from the glass production process. In the case where a glass using V₂O₅ as a raw material is produced in the same factory or a glass containing V₂O₅ is stored in the factory, a raw material of V₂O₅ or a cullet of a glass containing V₂O₅ is easily mixed in at a conveying line of the raw material or the cullet. In such a case, it is desirable to reduce V₂O₅ to be mixed in from the process as far as possible by no use of the line in which the raw material of V₂O₅ is handled at the time of conveying raw materials or batch blending or by keeping the conveying line and a storage place of the V₂O₅-containing glass cullet away from the production line of the Li₂O—Al₂O₃—SiO₂ based crystallized glass as far as possible.

In the Li₂O—Al₂O₃—SiO₂ based crystallized glass of the present invention, the V₂O₅ content is limited to from 0.08 to 15 ppm. As already mentioned, the V₂O₅ amount of 15 ppm or less can be achieved by devising raw materials, a glass composition, steps, and the like. When V₂O₅ is controlled to 15 ppm or less, a less colored Li₂O—Al₂O₃—SiO₂ based crystallized glass, especially a transparent Li₂O—Al₂O₃—SiO₂ based crystallized glass can be obtained. The content of V₂O₅ is preferably 10 ppm or less, more preferably 5 ppm or less, and further preferably 3 ppm or less.

Incidentally, even when complete prevention of the mixing of V₂O₅ from the process is succeeded, it is difficult to prevent the mixing of V₂O₅ from raw materials completely. When raw materials resulting in the V₂O₅ amount of 0.08 ppm or less are used, costs for the raw material drastically increase, so that it becomes difficult to obtain an inexpensive Li₂O—Al₂O₃—SiO₂ based crystallized glass. Accordingly, when the V₂O₅ content is defined to 0.08 ppm or more, preferably 0.1 ppm or more, further preferably 0.3 ppm or more, it becomes possible to provide a lower-cost and inexpensive Li₂O—Al₂O₃—SiO₂ based crystallized glass.

As₂O₃ or Sb₂O₃ are environmental burden substances and are not substantially contained in the present invention. Specifically, the content of each of As₂O₃ and Sb₂O₃ is 1,000 ppm or less, preferably 500 ppm or less, and further preferably 200 ppm or less.

SnO₂ is a component to be a substitute fining agent for As₂O₃ or Sb₂O₃. The content of SnO₂ is 0.01% or more, preferably 0.08% or more, and especially preferably 0.1% or more. An upper limit of the content of SnO₂ is 0.9%, more preferably 0.5% or less, more preferably 0.4% or less, and especially preferably 0.3% or less. When the content of SnO₂ is less than 0.01%, the effect as a fining agent is difficultly obtained. On the other hand, when the content of SiO₂ exceeds 0.9%, the coloration of V₂O₅ as a minute component becomes strong or the coloration of TiO₂ and Fe₂O₃ becomes too strong, so that the crystallized glass is liable to be colored yellowish and is prone to be easily devitrified.

Also, the Li₂O—Al₂O₃—SiO₂ based crystallized glass of the present invention preferably comprises a composition of from 55 to 75% of SiO₂, from 10 to 35% of Al₂O₃, from 1 to 10% of Li₂O, from 0.2 to 5% of MgO, from 0 to 5% of ZnO, from 0 to 10% of BaO, from 0 to 4% of TiO₂, from 0 to 5% of ZrO₂, from 0 to 4% of P₂O₅, and from 0.01 to 0.9% of SnO₂, in terms of % by mass. Particularly, the glass desirably comprises a composition of from 60 to 75% of SiO₂, from 17 to 27% of Al₂O₃, from 3 to 6% of Li₂O, from 0.2 to 4% of MgO, from 0 to 4% of ZnO, 3.7≦Li₂O+0.741MgO+0.367ZnO≦5.0, from 0.3 to 10% of BaO, from 0 to 2% of TiO₂, from 1 to 4% of ZrO₂, from 1 to 6% of TiO₂+ZrO₂, from 0 to 3% of P₂O₅, and from 0.01 to 0.3% of SnO₂, in terms of % by mass. The following will describe reasons why each component is defined as described above.

SiO₂ is a component that forms a skeleton of the glass and also constitutes a Li₂O—Al₂O₃—SiO₂ based crystal. The content of SiO₂ is preferably from 55 to 75%, more preferably from 58 to 70%, and especially preferably from 60 to 68%. When the content of SiO₂ is too small, the coefficient of thermal expansion tends to become high and it becomes difficult to obtain a crystallized glass excellent in thermal impact resistance. Also, chemical durability tends to decrease. On the other hand, when the content of SiO₂ is too large, melting ability of the glass becomes worse and the viscosity of a glass melt increases, so that there is a tendency that fining becomes difficult and forming of the glass becomes difficult.

Al₂O₃ is a component that forms a skeleton of the glass and also constitutes a Li₂O—Al₂O₃—SiO₂ based crystal. Moreover, Al₂O₃ can reduce, by its presence in the remaining glass phase of the crystallized glass, an increase in coloration due to SnO₂ and thus can diminish the coloration of the Li₂O—Al₂O₃—SiO₂ based crystallized glass to be a base. The larger the Al₂O₃ content in the glass composition is, the more the Al₂O₃ amount in the remaining glass phase after crystallization is, so that the coloration of the base glass can be reduced. Therefore, the content of Al₂O₃ is preferably 10% or more, more preferably 17% or more, further preferably 20% or more, especially preferably 20.5% or more, and most preferably 21.0% or more. When the content of Al₂O₃ is small, there is a tendency that the effect of reducing the increase in the coloration due to SnO₂ becomes difficult to obtain. Also, the coefficient of thermal expansion tends to become high and it becomes difficult to obtain a crystallized glass excellent in thermal impact resistance. Also, chemical durability tends to decrease. On the other hand, raw materials of Al₂O₃ frequently contain much V₂O₅ impurity. Moreover, when the content of Al₂O₃ is too large, melting ability of the glass becomes worse and the viscosity of a glass melt increases, so that there is a tendency that fining becomes difficult and forming of the glass becomes difficult. Furthermore, crystals of mullite precipitate and the glass tends to devitrify. Accordingly, an upper limit of the content of Al₂O₃ is preferably 35% or less, more preferably 30% or less, further preferably 27% or less, and especially preferably 25% or less.

Li₂O is a component that constitutes a Li₂O—Al₂O₃—SiO₂ based crystal, and is a component that exerts a large influence on crystallinity and also lowers the viscosity of the glass to improve glass melting ability and forming ability. The content of Li₂O is preferably 1% or more, more preferably 2% or more, further preferably 2.5% or more, and especially preferably 3% or more. When the content of Li₂O is too small, crystals of mullite precipitate and the glass tends to devitrify. Also, at the time of crystallization of the glass, the Li₂O—Al₂O₃—SiO₂ based crystal is less liable to precipitate and there is a tendency that a crystallized glass excellent in thermal impact resistance is difficult to obtain. Furthermore, melting ability of the glass becomes worse and the viscosity of a glass melt increases, so that there is a tendency that fining becomes difficult and forming of the glass becomes difficult. On the other hand, when the content of Li₂O is too large, the crystallinity becomes too strong, so that the glass tends to devitrify and the glass is liable to break. Accordingly, the content of Li₂O is preferably 10% or less, more preferably 6% or less, further preferably 5% or less, and especially preferably 4.5% or less, and most preferably 4% or less.

MgO is a component that exerts an influence on the coefficient of thermal expansion. For example, in the case where the glass is used in heat-resistant applications, in order to reduce a risk of breakage by thermal impact, the coefficient of thermal expansion is preferably close to 0 as far as possible. Accordingly, in the crystallized glass of the present invention, it is preferable that MgO is contained in an amount of 0.2% or more, particularly 0.5% or more. When MgO is contained in an amount of 0.2% or more, the coefficient of thermal expansion of the crystallized glass is liable to be close to 0. On the other hand, since a raw material of MgO tends to contain much V₂O₅ impurity, the mixing amount of V₂O₅ is liable to increase when the content of MgO is too large. Accordingly, the content of MgO is preferably 5% or less and more preferably 4% or less, and it is particularly preferable to limit the content to 2.9% or less.

ZnO is a component that exerts an influence on the coefficient of thermal expansion and its content is preferably from 0 to 5%, more preferably from 0 to 4%, and especially preferably from 0 to 3%. When the content of ZnO is too large, devitrification is liable to occur.

Furthermore, it is preferable to limit Li₂O+0.741MgO+0.367ZnO to 5.0% or less, particularly 4.8% or less, 4.6% or less, further 4.5% or less. When the value is small, the Al₂O₃ amount in the remaining glass phase in the crystallized glass is liable to be large and the coloration of the crystallized glass to be a base may be easily reduced. On the other hand, when the value of the above formula is too small, a grain diameter of the Li₂O—Al₂O₃—SiO₂ based crystal in the crystallized glass becomes large and there is a tendency that white turbidity is liable to generate. As a result, there is a concern that a transparent feeling of the crystallized glass is impaired, so that a lower limit thereof is preferably set to 3.7% or more.

BaO is a component that enhances a fining effect and, in the case of obtaining a white opaque glass, enhances whiteness of the glass. Its content is preferably from 0 to 10% and especially preferably from 0.3 to 10%. When the content of BaO is too large, the coefficient of thermal expansion tends to become large.

TiO₂ and ZrO₂ are nucleation agents. The contents of these nucleation agents are desirably strictly controlled for the following reasons. Namely, the larger the content of TiO₂ is, the more the crystal nuclei form and the less liable the generation of white turbidity is. On the other hand, when the content of TiO₂ is large, the coloration is liable to be strong. Moreover, the larger the content of ZrO₂ is, the more the crystal nuclei form and the less liable the generation of white turbidity is. On the other hand, when the content of ZrO₂ is large, devitrification tendency becomes strong and there is a tendency that a problem tends to occur in the forming step. Accordingly, an appropriate range of these nucleation agents are investigated in consideration of the Al₂O₃ amount, the amount of Li₂O+0.741MgO+0.367ZnO, and the like. As a result, the content of TiO₂ is preferably from 0 to 4%, more preferably from 1 to 3.5%, and especially preferably from 1 to 2.8% and the content of ZnO₂ is preferably from 0 to 5% and especially preferably from 1 to 4%. A lower limit of TiO₂+ZrO₂ (total amount of TiO₂ and ZrO₂) is preferably 1% and especially preferably 3%. Moreover, an upper limit of TiO₂+ZrO₂ is preferably 9% and particularly desirably 6%.

P₂O₅ is a component that promotes phase separation of the glass and makes the precipitation of the Li₂O—Al₂O₃—SiO₂ based crystal easy. Its content is preferably from 0 to 4% and particularly desirably from 0 to 3%. When the content of P₂O₅ is too large, white turbidity is liable to generate and the coefficient of thermal expansion tends to increase.

When Nd₂O₃ and CoO that are colorants are used, the coloration can be reduced due to the effect of complementary colors. However, Nd₂O₃ and CoO are rare resources and cost high, so that it becomes difficult to provide a low-cost and inexpensive Li₂O—Al₂O₃—SiO₂ based crystallized glass when these components are used. Moreover, transmittance in a visible region is lowered and the appearance looks blackish, and a transparent feeling tends to be impaired. Therefore, the contents of Nd₂O₃ and CoO are preferably each less than 500 ppm, more preferably each less than 300 ppm, and further desirably each less than 100 ppm.

Also, as for Fe₂O₃ that mixes in as an impurity component, the content is desirably limited. Specifically, the content of Fe₂O₃ is preferably 250 ppm or less and especially preferably 200 ppm or less. As for Fe₂O₃, a smaller content is more preferable, since coloration decreases. However, for example, in order to control the content to less than 50 ppm, it is necessary to use highly pure raw materials and it becomes difficult to provide an inexpensive Li₂O—Al₂O₃—SiO₂ based crystallized glass. Accordingly, the content of Fe₂O₃ is desirably 50 ppm or more.

In addition, in the Li₂O—Al₂O₃—SiO₂ based crystallized glass of the present invention, color tone of a transmitted light at a thickness of 3 mm is preferably 4.5 or less, more preferably 4.0 or less, and especially preferably 3.5 or less in terms of the b* value of L*a*b* indication of CIE standard. Also, transmittance of a light having a wavelength of 525 nm is preferably 89% or more and especially preferably 89.5% or more at a thickness of 1.1 mm.

Furthermore, since the Li₂O—Al₂O₃—SiO₂ based crystallized glass of the present invention is used in heat-resistance applications, the coefficient of thermal expansion is preferably close to zero as far as possible. Specifically, the coefficient is preferably from −2.5×10⁻⁷/° C. to 2.5×10⁻⁷/° C. and especially preferably from −1.5×10⁻⁷/° C. to 1.5×10⁻⁷/° C. in a temperature range of from 30 to 380° C. When the coefficient of thermal expansion is out of the range, a risk of breakage by thermal impact is liable to increase.

The following will describe the method for producing a Li₂O—Al₂O₃—SiO₂ based crystallized glass of the present invention.

First, a batch is prepared so as to be a target glass composition. Since the target composition is as already described, an explanation thereof is omitted.

Here, it is necessary to select a raw material of Al₂O₃, a raw material of MgO, and the like each having a small V₂O₅ content so that the V₂O₅ amount contained in a glass becomes from 0.08 to 15 ppm. In addition, contamination with V₂O₅ from the process is excluded as far as possible by managing the steps. For example, dedicated equipments are used for compounding and conveying a raw material batch and a place of installing a line is devised so that no influence is exerted from production facilities of a V₂O₅ containing glass.

Then, the glass raw material batch is melted. A degree of the coloration of the crystallized glass is influenced by not only the glass composition but also melting conditions. Particularly, in the case of adding SnO₂, the coloration tends to become strong when the molten glass goes toward a reduction direction. The reason is considered that Sn²⁺ has a large influence on the coloration as compared with Sn⁴⁺. In order that the molten glass may not go toward the reduction direction as far as possible, it is preferable that melting temperature is lowered or melting time is shortened. For the melting time, melting efficiency (melting area/flow rate) can be adopted as its index. Thus, by limiting the melting temperature and the melting efficiency, it becomes possible to prevent the molten glass from going toward the reduction direction to obtain a crystallized glass having a reduced coloration.

Maximum temperature at the time of glass melting is preferably less than 1,750° C. and especially preferably 1,700° C. or less. When the maximum temperature at the time of glass melting is 1,750° C. or more, a component of Sn is liable to be reduced and the coloration tends to become strong. The phenomenon tends to occur more remarkably particularly when V₂O₅ is present as an impurity. A lower limit of the maximum temperature at the time of glass melting is not particularly limited but is preferably 1,600° C. or more and especially preferably 1,650° C. or more in order to advance a glass reaction sufficiently to obtain a homogeneous glass.

The melting efficiency of the glass is preferably from 1 to 6 m²/(t/day), and especially preferably from 1.5 to 5 m²/(t/day). When the melting efficiency of the glass is less than 1 m²/(t/day), the melting time is shortened and as a result fining time is also shortened, so that there is a tendency that a glass excellent in bubble quality is difficult to obtain. On the other hand, when the melting efficiency of the glass exceeds 6 m²/(t/day), a component of Sn is liable to be reduced and the coloration tends to become strong.

Subsequently, by forming the molten glass into a predetermined shape, a Li₂O—Al₂O₃—SiO₂ based crystallizable glass can be obtained. Here, as a forming method, various forming methods such as a float method, a press method, a roll-out method, and an overflow method can be applied depending on an objective shape.

Thereafter, the formed Li₂O—Al₂O₃—SiO₂ based crystallizable glass was subjected to a heat treatment at from 600 to 800° C. for from 1 to 5 hours to form crystal nuclei (crystal nuclei-forming stage) and further subjected to a heat treatment at from 800 to 950° C. for from 0.5 to 3 hours to precipitate a Li₂O—Al₂O₃—SiO₂ based crystal as a main crystal (crystal-growing stage), thereby obtaining a transparent crystallized glass containing a β-quartz solid solution (Li₂O.Al₂O₃.nSiO₂ [4>n≧2]) as a main crystal. Incidentally, when the Li₂O—Al₂O₃—SiO₂ based crystallizable glass is subjected to a heat treatment at from 600 to 800° C. for from 1 to 5 hours to form crystal nuclei and then further subjected to a heat treatment at from 1,050 to 1,250° C. for from 0.5 to 3 hours, a white crystallized glass containing a 3-spodumene solid solution (Li₂O.Al₂O₃.nSiO₂ [n≧4]) as a main crystal can be also obtained.

The thus prepared Li₂O—Al₂O₃—SiO₂ based crystallized glass of the present invention is provided for various applications after being subjected to post processing such as cutting, polishing, or bending processing, or being subjected to decoration on the surface.

EXAMPLES

The present invention is hereunder described with reference to Examples but the present invention should not be construed as being limited to the following Examples.

TABLE 1 Glass composition Sample No. (% by mass) 1 2 3 4 5 6 SiO₂ 65.5 65.4 65.6 65.5 65.2 65.5 Al₂O₃ 22.3 22.2 22.4 22.4 21.9 22.3 Li₂O 3.7 3.7 3.7 3.7 4.2 3.7 Na₂O 0.4 0.4 0.4 0.4 0.4 0.4 K₂O 0.3 0.3 0.3 0.3 0.3 0.3 MgO 0.7 0.7 0.7 0.7 0.9 0.7 ZnO BaO 1.2 1.2 1.2 1.2 1.2 1.2 TiO₂ 2.0 2.0 2.0 2.0 2.0 2.0 ZrO₂ 2.2 2.2 2.2 2.2 2.2 2.2 P₂O₅ 1.4 1.4 1.4 1.4 1.4 1.4 SnO₂ 0.3 0.5 0.1 0.2 0.3 0.3 Fe₂O₃ (ppm) 100 100 100 100 100 100 V₂O₅ (ppm) 2 1 3 0.3 3 2 Nd₂O₃ — — — — — — Li + 0.741Mg + 0.367Zn 4.2 4.2 4.2 4.2 4.9 4.2 Melting temperature (° C.) 1680 1680 1680 1680 1680 1750 Color tone b* 3.0 2.9 3.2 2.5 2.5 3.8 Transmittance (%) 89.9 89.9 89.9 90.2 89.8 89.2 Coefficient of thermal expansion −0.9 −0.9 −0.9 −0.9 −1.1 −0.9 (×10⁻⁷/° C.)

TABLE 2 Glass composition Sample No. (% by mass) 7 8 9 10 11 SiO₂ 65.5 65.2 65.2 65.2 65.2 Al₂O₃ 22.3 21.9 22.3 21.9 21.9 Li₂O 3.7 3.7 3.7 4.2 4.2 Na₂O 0.4 0.4 0.4 0.4 0.4 K₂O 0.3 0.3 0.3 0.3 0.3 MgO 0.7 0.7 0.7 0.9 0.9 ZnO BaO 1.2 1.2 1.2 1.2 1.2 TiO₂ 2.0 2.0 2.0 2.0 2.0 ZrO₂ 2.2 2.2 2.2 2.2 2.2 P₂O₅ 1.4 1.4 1.4 1.4 1.4 SnO₂ 0.3 1.0 0.3 — — Fe₂O₃ (ppm) 100 100 100 100 100 V₂O₅ (ppm) 20 5 20 20 2 Nd₂O₃ — — 0.2 — — As₂O₃ — — — 0.5 0.5 Li + 0.741Mg + 0.367Zn 4.2 4.2 4.2 4.2 4.2 Melting temperature 1680 1680 1680 1680 1680 (° C.) Color tone b* 8.2 4.5 4.0 3.5 2.2 Transmittance (%) 87.3 88.9 86.2 89.6 90.3 Coefficient of thermal −0.9 −0.9 −0.9 −1.0 −1.0 expansion (×10⁻⁷/° C.)

Table 1 shows Examples of the present invention (Sample Nos. 1 to 6) and Table 2 shows Comparative Examples (Sample Nos. 7 to 9). Incidentally, Sample Nos. 10 and 11 are Reference Examples.

First, respective raw materials were compounded each in a form of an oxide, a hydroxide, a carbonate salt, a nitrate salt, or the like and homogeneously blended. Here, with regard to the raw materials of Al₂O₃, MgO, ZrO₂, and SiO₂, V₂O₅ amounts of various raw materials were previously confirmed by chemical analysis on ICP-AES and there were used raw materials in which the V₂O₅ amount was confirmed to be detection limit or less (detection limit: 0.07 ppm). Furthermore, the adjustment of the V₂O₅ amount was conducted by adding a predetermined amount of vanadium pentoxide (V₂O₅). In addition, as the formulation equipment and conveying equipment of the raw material batch, there were used equipments in which any V₂O₅-containing glass was not handled in the past.

Subsequently, the resulting raw material batch was charged into a refractory furnace fitted with an oxygen combustion burner and was melted at a melting efficiency of 2.5 m²/(t/day) at a maximum temperature in the table. After the glass melt was stirred with a platinum stirrer, the melt was roll-formed in a thickness of 4 mm and further cooled to room temperature within an annealing furnace, thereby obtaining a crystallizable glass.

After the crystallizable glass was subjected to a heat treatment at 760 to 780° C. for 3 hours to perform nucleation, the glass was further subjected to a heat treatment at 870 to 890° C. for 1 hour to achieve crystallization, thereby obtaining a transparent crystallized glass. For the obtained crystallized glass, the V₂O₅ amount, color tone, transmittance, and coefficient of thermal expansion were measured.

The V₂O₅ amount was confirmed by chemical analysis using ICP-AES.

The color tone of a transmitted light was evaluated by measuring transmittance in a wavelength range of from 380 to 780 nm using a spectrophotometer and calculating an L*a*b* value of CIE Standard from the transmittance.

The transmittance was evaluated by transmittance at a wavelength of 525 nm measured using a spectrophotometer, for a crystallized glass plate, both sides of which was subjected to optical polishing into a thickness of 1.1 mm.

The coefficient of thermal expansion was evaluated by an average linear coefficient of thermal expansion measured in a temperature region of from 30 to 380° C. using a glass sample processed into a solid bar of 20 mm×5 mmφ.

As is apparent from Table 1, it is revealed that the crystallized glasses of Examples all have a b* value of as small as 4.5 or less and a transmittance of as high as 89% or more.

While the present invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the present invention.

The present application is based on a Japanese patent application filed on May 31, 2012 (Japanese Patent Application No. 2012-124081), the contents of which are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

The Li₂O—Al₂O₃—SiO₂ based crystallized glass of the present invention is suitable for front windows of oil stoves, wood stoves, and the like, substrates for high-technology products such as color filter substrates and image sensor substrates, setters for firing electronic components, trays for microwave ovens, top plates for electromagnetic cooking, window glasses for fire-retarding doors, and the like. 

The invention claimed is:
 1. A method for producing a Li₂O—Al₂O₃—SiO₂ based crystallized glass, which comprises preparing a raw material batch so as to form a glass comprising from 0.01 to 0.9% of SnO₂ in terms of % by mass and having a content of each of As₂O₃ and Sb₂O₃ of 1,000 ppm or less, melting and forming the batch to produce a Li₂O—Al₂O₃—SiO₂ based crystallizable glass, and subsequently subjecting the glass to a heat treatment to achieve crystallization, wherein the b* value of L*a*b* colorimetric system of CIE standard of the crystallized glass standard is from 0 to 4.5 at a thickness of 3 mm and wherein selection of raw materials and management of steps are performed so that a V₂O₅ content in the crystallized glass becomes from 0.08 to 15 ppm.
 2. The method for producing a Li₂O—Al₂O₃—SiO₂ based crystallized glass according to claim 1, which comprises preparing the batch so as to form a glass comprising from 55 to 75% of SiO₂, from 10 to 35% of Al₂O₃, from 1 to 10% of Li₂O, from 0.2 to 5% of MgO, from 0 to 5% of ZnO, from 0 to 10% of BaO, from 0 to 4% of TiO₂, from 0 to 5% of ZrO₂, from 0 to 4% of P₂O₅, and from 0.01 to 0.9% of SnO₂, in terms of % by mass, as a glass composition.
 3. The method for producing a Li₂O—Al₂O₃—SiO₂ based crystallized glass according to claim 2, wherein a raw material having a V₂O₅ content of 50 ppm or less is used as a raw material of Al₂O₃.
 4. The method for producing a Li₂O—Al₂O₃—SiO₂ based crystallized glass according to claim 2, wherein a raw material having a V₂O₅ content of 400 ppm or less is used as a raw material of MgO.
 5. The method for producing a Li₂O—Al₂O₃—SiO₂ based crystallized glass according to claim 2, wherein the batch is prepared so as to form a glass comprising from 17 to 27% of Al₂O₃ and from 0.2 to 4% of MgO, in terms of % by mass, as a glass composition.
 6. The method for producing a Li₂O—Al₂O₃—SiO₂ based crystallized glass according to claim 1, wherein melting is performed at a temperature of less than 1,750° C.
 7. The method for producing a Li₂O—Al₂O₃—SiO₂ based crystallized glass according to claim 1, wherein a β-quartz solid solution is precipitated as a main crystal by a heat treatment. 